1. Field of the Invention
The present invention relates generally to an induction heating apparatus and, more particularly, is directed to an induction heating apparatus for cooking.
2. Description of the Prior Art
There has recently been proposed an induction heating apparatus for cooking in which an object, such as a pan or the like, which is made of a ferromagnetic material, is placed in a high frequency time-varying magnetic field. As a result, the object is heated by eddy current loss generated therein. In such heating apparatus, the object to be heated is placed on a so-called top plate made of an insulated material and is then heated as described above. In this manner, the top plate itself is not heated and there is little risk of the user being burned thereby.
It should be appreciated that such eddy current loss is greater in objects made of a ferromagnetic material. With the above induction heating apparatus for cooking, if a pan not made of a ferromagnetic material, such as aluminum or copper, and thereby constituting a light induction load, is placed on the top plate, no or little eddy current loss is generated in the pan. Thus, the pan is not heated even though an AC current is supplied to an induction heating coil for generating the time-varying magnetic field. This, of course, results in the supply current being unnecessarily consumed. It is therefore necessary to inhibit the heating operation, that is, the supply of AC current to the induction heating coil, for non-ferromagnetic objects which are not suitable for an induction heating operation.
In order to avoid the above problem, it has been proposed to provide a magnetic material detector which detects whether the object to be heated is made of a ferromagnetic material. The magnetic material detector controls the supply of current to the induction heating coil so that only objects which are made of a ferromagnetic material are heated.
However, it may be desirable to use such induction heating apparatus to heat objects made of materials which present a sufficiently heavy induction load so as to be suitable for an induction heating operation. For example, stainless steel 18-8 is less adapted for induction heating than iron, although it is more adapted for induction heating than aluminum or copper. Also, a very thin non-ferromagnetic metal container, for example, made from aluminum foil, which is sufficiently thin in comparison to the skin depth, can be heated by an induction heating operation due to the skin effect.
However, in the aforementioned known induction heating apparatus, the magnetic material detector prevents the heating of such objects made of stainless steel 18-8 and aluminum foil since such objects are not made of a ferromagnetic material. Even if the magnetic material detector can be controlled or overridden so as to cause the induction heating coil to heat such objects made of, for example, stainless steel 18-8 and aluminum foil, a problem may result during later use if an object made of a non-ferromagnetic metal material, such as stainless steel 18-8 and aluminum foil, which is not to be heated is subsequently placed on the top plate. For example, if a switch is used to override the magnetic material detector so as to heat a stainless steel 18-8 pan, and if the switch is negligently left in its operative position after the induction heating operation, there is a danger of excessively heating a second stainless steel 18-8 pan later placed on the top plate by mistake.